Investigation of the Influence of the Application Path of Confining Pressure: Tests on Actively Confined and FRP-Confined Concretes

It is often assumed that, at a given lateral strain, the axial compressive stress and strain of fiber reinforced polymer (FRP)–confined concrete are the same as those of the same concrete when it is actively confined under a confining pressure equal to that supplied by the FRP jacket. An experimental program was undertaken to assess the validity of this assumption, in which 63 actively confined and FRP-confined normal-strength (NSC) and high-strength concrete (HSC) specimens were tested under axial compression. The axial stress-strain and lateral strain-axial strain curves obtained from the two different confinement systems were assessed. The results indicate that, at a given axial strain, lateral strains of actively confined and FRP-confined concretes correspond when they are subjected to the same lateral confining pressure. At the points of intersection on the lateral strain-axial strain curves, FRP-confined NSC exhibits only slightly lower axial compressive stresses compared with those of actively confined NSC. Conversely, the difference between the axial stresses of actively confined and FRP-confined HSC is found to be significant, indicating that the compressive behavior of confined HSC is more sensitive to the application path of confining pressure than the behavior of confined NSC. By using the combined results of the present study and two comprehensive experimental databases of actively confined and FRP-confined concretes, an expression has been developed for the prediction of the difference in the confining pressures that results in differences in the axial stresses between actively confined and FRP-confined concretes.